Adjusting the neck on your guitar
One of the most common adjustments that your guitar needs is a neck adjustment. A neck adjustment involves turning the nut on the end of the truss rod to set the neck relief properly. First, some definition of terms:
1. Truss rod – a metal torque rod that is set in the neck of the guitar before the fingerboard is glued on to the front of the neck. The nut that adjusts this rod is found either under a little cover at the bottom front of the headstock, or inside the sound hole at the spot where the neck joins the body.
2. Adjustment wrench – the tool used for this adjustment is either an allen wrench of the proper size (may have come with the guitar) if the the end of the rod is a female type or a socket wrench that fits over the nut.
3. Neck relief – the amount of deviation from “dead flat” to “bowed out” of the neck in relation to the straight line of the strings. It also could be bowed in the direction of the strings. If the neck is bowed too far away from the strings the action can be unplayably high. If the neck is bowed toward the strings the strings may be too close to the frets to ring out properly resulting in buzzing.
4. Sighting the neck – the way one determines if the neck relief is correct is to sight down the fingerboard and looking at the edge of the fingerboard compare it to the strings using the strings as a straight edge. This takes some experience to understand what you are looking at but basically if the neck is bowed away from the strings too much the truss rod can be tightened (clockwise) to straighten it out. If it is bowed toward the strings it can be loosened (counter-clockwise) to introduce some relief. Usually a small amount of relief is needed in order for the guitar to play cleanly. There are more technical ways to determine the relief such as fretting the 6th string at the first and 12th frets and measuring the distance from string to fret about half way between. The relief should be between .005” and .012” depending on the guitar and the players style. Someone with a light touch can use a lower action and someone who plays harder may need higher action. No one measurement works for every guitar.
5. Action – the action of the guitar is the distance between the bottom of the string and the top of the frets. It is generally a bit closer near the nut than it is higher up on the fingerboard.
The amount you turn the truss rod nut is usually very little, if you turn more that a half turn you may have other problems. These adjustments are usually very small and may be required as seasons change and more often if your guitar is lightly built like many fine instruments. It is a good skill to master so you take care of your own instrument in this regard.
Good luck and learn to carefully sight your neck to determine it’s condition and proceed with caution and the proper tools.
Check out this link for a more detailed description and pictures: